The broad objectives of this research are to clarify the role of the western gray squirrel (Sciurus griseus) as a keystone species for maintaining enzootic foci of the Lyme disease (LD) spirochete Borrelia burgdorferf sensu stricto (Bb ss) in the far-western USA; to investigate the host-seeking behavior of Ixodes pacificus (Ip) nymphs in relation to environmental parameters and to risk of human exposure to Bb ss; to ascertain what genospecies of Bb sensu lato (si) and other emerging bacterial zoonotic agents (e.g., Anaplasma phagocytophilum [Ap]) infect humans and their clinical correlates; and to complete the molecular and modeling components of an ongoing study to predict the risk of human exposure to Ip nymphs in dense woodlands of northwestern California. The reservoir competence of S. griseus for Bb ss and Ap will be evaluated by determining the infectivity of naturally infected squirrels for uninfected Ip larvae; the capacity of fed larvae to pass infection transstadially; the ability of infected nymphs to transmit infection to naive squirrels; the duration of infectivity in experimentally infected squirrels; and, in part, the geographic range of infected squirrels. The host-seeking activities of Ip nymphs in relation to biotic and abiotic factors will be investigated in woodlands where humans are at elevated risk of exposure to this life stage. These will include the diurnal questing cycle; the densities of host-seeking nymphs, and of Bb ss- or Ap-infected nymphs, on logs and tree trunks versus adjacent leaf litter; and the movements of marked ticks, and the frequency with which lizards encounter them after their release on logs versus leaf litter. The high diversity of Bb si in the Far West begs the question, which genospecies besides Bb ss can infect and cause human illness? Accordingly, serum specimens from hundreds of LD patients will be assayed molecularly for Bb si and co- infections with Ap or Bartonella spp. A stepwise multiple regression model for human exposure to Bb ss- infected Ip nymphs will be created after -5,500 Ip nymphs collected from 78 woodland sites have been tested by PCR. Development of this risk model, which culminates over 2 decades of intensive study, should contribute significantly to our knowledge of the landscape ecology and epidemiology of LD in this region.